U.S. Pat. No. 4,469,679 describes the preparation of the target vasopressin antagonist by oxidation of a linear 1,6-dimercapto-containing peptide using various mild oxidizing agents such as ferricyanide, oxygen or iodine. The use of oxygen involves passage of the gas through the reaction mixture "for several days." Also, extremely dilute reaction conditions are there described as preferred, column 2 lines 47-56. A number of other publications have similar disclosures for preparing vasopressin-like compounds, for example, Manning et al., U.S. Pat. No. 4,367,225 or Ali et al., U.S. Pat. No. 4,481,194.
Another series of publications, such as Cavallini et al., Archives of Biochemistry and Biophysics 130, 354-361, describes the intermolecular oxidation of cysteine to the dimeric cystine by using a copper catalyzed oxidation with the cysteine-Cu.sup.II complex being the intermediate catalyst.
As far as we are aware, copper II-catalyzed oxidation has not been applied to form an intramolecular S-S bond previously.